Consider the molecule 4-(2-hydroxyethyl)phenol. When this compound is treated with one equivalent of a dilute strong acid, which site is preferentially protonated?

Step-by-step solution

The molecule 4-(2-hydroxyethyl)phenol contains two types of hydroxyl groups: a phenolic -OH and an aliphatic primary -OH . Protonation requires the availability of a lone pair of electrons on the oxygen atom. In the phenolic -OH group, the lone pairs on oxygen are delocalized into the benzene ring through resonance ( +R effect), making them less available to accept a proton. In the aliphatic -OH group, the lone pairs are localized and their electron density is slightly enhanced by the +I effect of the alkyl chain.